Carbon nanotubes (CNTs) offer significant advantages over other materials in that they possess substantially higher strength-to-weight ratio and superior mechanical properties. A major limitation to their large-scale commercialization however, has remained the need for large quantity, cost-effective production methods. Conventional synthetic methods for synthesis of CNTs utilize arc discharge, laser ablation and chemical vapor deposition (CVD). Existing manufacturing methods using CVD are mainly directed toward obtaining aligned monolayer arrays of CNTs on a catalyst surface that is comprised of either a metallic substrate, or a non-metallic substrate whose surface is coated with a metallic material.
Metal catalysts for CNT synthesis disclosed in the art involve the deposition of a transition metal catalyst layer as a coating on a substrate by standard methods such as metal vapor deposition and magnetron sputtering. Such methods involve a combination of metallic (non-catalytic) and non-metallic substrates coated with a surface layer of a catalytic metal such as iron. They however, require relatively expensive and complex reactor apparatus, and typically require a high vacuum (10−5 to 10−7 torr) environment. Furthermore, such methods are only capable of providing a uniform flat surface layer of the metal catalyst on the substrate on which CNT formation and growth can occur. The surface area of the catalytic metal layer therefore, is substantially similar to that of the substrate on which it is deposited. Since CNT yield is directly related to surface area of the catalytic surface, substantially large areas of metal coated substrate is required to synthesize large quantities of CNTs, which is impractical in terms of existing limitations of the reaction apparatus.
A mesoporous silica sol-gel catalyst impregnated with iron was disclosed by Li et al. (Science, Vol. 274, (1996), 1701) for the synthesis of aligned carbon nanotubes. The method described by Li requires the preparation of large, flat surfaces of the iron impregnated mesoporous silica substrates with uniform distribution of pores. According to Li et al., preparation of such large area catalytic substrate is hampered by the inherent tendency to shrink, crack and shatter during their preparation. Meticulous drying procedures therefore, are required to maintain the integrity of the catalyst to obtain large area surfaces, which is critical for obtaining high density monolayer CNT arrays. Imperfect catalyst preparation can severely limit yields of CNT product. Also, CNT synthesis by the process of Li et al. requires a reaction temperature of 700° C., which is impractical for substrates such as flat panel glass. Methods for producing an aligned array of linear CNTs on a substrate surface has been described in WO 99/65821 by Ren et al. in plasma conditions under an applied electrical field. Such methods however, require high vacuum conditions, which is difficult to achieve in large reactors in a commercially viable CNT manufacturing processes.
Although such methods are capable of providing highly pure, aligned CNTs, they are not best suited for large-scale production due to low volume (typically several milligrams to grams per batch), low yields based on amount of catalyst and high manufacturing cost. Furthermore, existing methods do not allow control of nanotube morphology, tubule diameter, tubule wall thickness and other structural elements that are important in achieving desired material properties that may required for specific applications. Such drawbacks are limiting factors that restrict the widespread use of CNTs in potential applications.
Most of the prior art methods provide methods for synthesis of linear CNTs without morphology control. However, the anomalous electrical properties exhibited by “kinked” or bent CNT tubules is indicative of the importance of non-linear, branched tubule structures in the development of CNT based electronic devices such as micro-transistors and nanocircuits. Although it is theoretically possible to introduce a wide range of structural defects with useful electronic properties in CNTs, synthetic limitations have precluded such introduction of systematic structural defects. Furthermore, currently available methods do not allow controlled alteration of linear tubule structures during their growth. Post growth modifications of CNTs have been difficult to implement and are prone to uncontrolled and random defects. Li et al disclose a method to synthesize a CNT with a branched Y-junction (Nature, (1999) Vol. 402, 253–4) that involves the deposition of carbon onto an thin aluminum sheet wherein Y-shaped molds are etched by an electrochemical process. The CNTs formed within the aluminum molds are then removed from within the said molds. The branched Y-shaped CNTs obtained by the method however, are not symmetrical with respect to arm length, straightness and angles between arms, since their shape and symmetry is determined by limitations in fabrication of the aluminum mold in which they are formed. Such processes are also not suited for large scale manufacture of CNTs and are, therefore, not economically viable for use in a commercial process.